Thousands of people crowded an Alabama bridge on Sunday to commemorate a bloody confrontation 50 years ago between police and peaceful protesters that was a landmark in the civil rights movement and helped bring about the 1965 Voting Rights Act.

A day after U.S. President Barack Obama had walked atop the Edmund Pettus Bridge, many jammed shoulder to shoulder, many unable to move, as they recalled the civil rights struggle.

Police said at least 15,000 to 20,000 people had joined the crush on and around the small bridge. Many came from around the country for several events commemorating the event.

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William Baldwin, 69, of Montgomery, Ala., brought his two grandsons, ages 11 and 15, to the bridge Sunday so they could grasp the importance of the historic march he took part in a half century earlier.

"They're going to take this struggle on and we have to understand the price that was paid for them to have what they have now," Mr. Baldwin said. "It wasn't granted to them. It was earned by blood, sweat and tears."

Some sang hymns and others held signs, such as "Black lives matter, all lives matter." The crowd was so large that many appeared barely able to move as they peacefully sought to make their way across.

On March 7, 1965, police beat and tear-gassed marchers at the foot of the bridge in Selma in a spasm of violence that shocked the United States. The attack help build momentum for Congress to pass the Voting Rights Act later that year.

Earlier Sunday, Selma officials paid tribute to the late President Lyndon B. Johnson for the Voting Rights Act. The attack on demonstrators preceded a Selma-to-Montgomery march, which occurred two weeks later in 1965.

Luci Baines Johnson accepted the award on behalf of her father, saying it meant so much to see him honoured.

"You remember how deeply Daddy cared about social justice and how hard he worked to make it happen," she told a crowd Sunday morning. Several hundred people gave her a standing ovation and some chanted, "L.B.J., L.B.J."

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She said what happened in Selma changed the world, adding that she witnessed the painful injustice of segregation as a child in Texas.

An anniversary march from Selma to Montgomery is set to begin Monday morning and culminate with a rally at the Alabama Capitol Friday afternoon.

On Saturday, Mr. Obama joined civil rights leaders and others at the bridge and talked about progress in race relations since the 1960s. He mentioned recent high-profile clashes between citizens and law enforcement on the circumstances leading to police killings of unarmed black men and law enforcement tactics toward minorities.

"We just need to open our eyes, and ears, and hearts, to know that this nation's racial history still casts its long shadow upon us," Mr. Obama said. "We know the march is not yet over, the race is not yet won, and that reaching that blessed destination where we are judged by the content of our character requires admitting as much."

Mr. Obama was joined by others, including Congressman John Lewis of Georgia – an Alabama native who was among the demonstrators attacked by law officers on the March 7, 1965, march for equal voting rights.

Bishop Dennis Proctor of the Alabama-Florida Episcopal District said his group brought five buses to the anniversary commemoration. But he told members not to come to Selma if they couldn't commit to fighting to restore protections in the Voting Rights Act that were recently eliminated.

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The U.S Supreme Court in 2013 struck down Section 4 of the Voting Rights Act which required states with a history of minority voter suppression to get permission from the Justice Department before changing voting laws.

The Rev. Al Sharpton, a prominent civil rights activist who spoke at Sunday's unity breakfast, said recent changes in state voting laws threatened to push minority voters backward down the bridge.

"While we are celebrating, there are those that are trying to dismantle what we are celebrating," Rev. Sharpton said.

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